Seasonal changes in picoplankton composition in the Northern Adriatic Sea as assessed by CARD-FISH and fingerprinting

There has been a virtual explosion of studies in recent years that use molecular biological techniques to analyze the community composition and function of marine microbial communities. These techniques permit characterizations without the need to cultivate the organisms, which opens up a much larger fraction of the community than traditional cultivation-based methods. Among the new techniques, there is cloning and comparative sequencing of taxonomically useful markers like 16S rRNA, community fingerprinting approaches like denaturing gradient gel electrophoresis (DGGE) and terminal restriction fragment length polymorphism (T-RFLP), and fluorescence in situ hybridization (FISH).

The picoplankton, composed of Bacteria and Archaea, plays a significant role in marine food webs and in most biogeochemical cycles in marine systems. The study of picoplankton diversity in marine habitats has been strongly advanced with the recent introduction of molecular tools. The taxonomic composition of assemblages and their temporal and spatial dynamics in the oceans are of major importance for global element cycling. This is reflected in the rapid development of the research field of marine biogeochemistry. Seasonal variations of bacterial community have been observed in marine, estuarine and freshwater ecosystems. Physiological predisposition and nutritional tolerance of dominant bacteria tend to maintain stable communities during certain season. Meanwhile, authors have reported that bacteria are likely influenced by abiotic characteristics (salinity, pH, water clarity, substrates resources, phytoplankton and chlorophyll a concentration) and microbial food web structures (grazing and viral lysis) of aquatic systems. The temporal variation in picoplankton communities is caused by bottom-up and top-down control, and overall a complex ecological process. A causative understanding of this complex microbial network is, however, required to understand the consequences of large-scale or even global events like the recent oil disaster in the Gulf of Mexico or the raising level of atmospheric concentrations of carbon dioxide.

The Adriatic Sea is the northeast part of the Mediterranean Sea. The shallow northern Adriatic is characterized by high seasonal and long-term variability of oceanographic and biological conditions, mainly due to climatic fluctuations and external nutrient input principally from the Po River, one of major rivers in the Mediterranean. Phosphorus depletion and the high N:P ratio in the Adriatic Sea has been frequently suggested to induce an accumulation of mucilage. In the north part of the Adriatic Sea there are strong physical, chemical and biological gradients, which provides a representative ecosystem to study the dynamics of bacterioplankton communities.

The proposed project aims at an in-depth characterization of picoplakton community in the northern Adriatic Sea using in situ localization and quantification by CARD-FISH. A special focus will be on the study of snow particle that are very often present in this basin. We also plan to establish a new collaboration between the German and Croatian institution and transfer of the knowledge between the two partner institutes.